package com.tx.blog.util;

/**
 * @author ：tx
 * @description：id生成  twitter雪花算法
 * @date ：Created in 2019/4/24 11:15
 * @modified By：
 * @version:
 */
public class IdGenerator {
    private long workerId;
    private long dataCenterId;
    private long sequence = 0L;
    /**
     * 该项是Thu, 04 Nov 2010 01:42:54 GMT) 这一时刻到1970-01-01 00:00:00时刻所经过的毫秒数
     */
    private long twEpoch = 1288834974657L;
    private long workerIdBits = 5L;

    /**
     * 最后一次修改时间
     */
    private long lastTimestamp = -1L;

    /**
     * 节点ID长度
     */
    private long dataCenterIdBits = 5L;

    /**
     * 最大支持机器节点数0~31，一共32个
    */
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    /**
     * -1=补码=反码+1
     *     原码：     1000 0001
     *     反码：     1111 1110
     *     补码：     1111 1111
     *     -1<<5      1110 0000
     *  -1^ (-1<<5)   0001 1111  == 31
     */

    /**
     * 最大支持数据中心节点数0~31，一共32个
     */
    private long maxDataCenterId = -1L ^ (-1L << dataCenterIdBits);

    /**
     * 序列号12位
     */
    private long sequenceBits = 12L;

    /**
     * 最大为4095
     */
    private long sequenceMask = -1L ^ (-1L << sequenceBits);



    /**
     * 机器节点左移12位
     */
    private long workerIdShift = sequenceBits;

    /**
     * 数据中心节点左移17位
     */
    private long dataCenterIdShift = sequenceBits + workerIdBits;

    /**
     * 时间毫秒数左移22位
     */
    private long timestampLeftShift = sequenceBits + workerIdBits + dataCenterIdBits;



    private static class IdGenHolder {
        private static final IdGenerator instance = new IdGenerator();
    }

    public static IdGenerator get() {
        return IdGenHolder.instance;
    }

    public IdGenerator() {
        this(0L, 0L);
    }

    public IdGenerator(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDataCenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDataCenterId));
        }
        this.workerId = workerId;
        this.dataCenterId = datacenterId;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();
        //获取当前毫秒数
        //如果服务器时间有问题(时钟后退) 报错。
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format(
                    "Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        //如果上次生成时间和当前时间相同,在同一毫秒内
        if (lastTimestamp == timestamp) {
            //sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
            sequence = (sequence + 1) & sequenceMask;
            //判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
            /**
             *  类似于
             *  000000000 4096在后12位的样子 进1了
             *  111111111 4095
             *  000000000 相于
             */
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
                //自旋等待到下一毫秒
            }
        } else {
            sequence = 0L;
            //如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
    }
        lastTimestamp = timestamp;
        // 最后按照规则拼出ID。
        // 000000000000000000000000000000000000000000  00000            00000       000000000000
        // time                                      datacenterId      workerId     sequence
        // return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
        //        | (workerId << workerIdShift) | sequence;
        //时间-数据中心（进程）--机器编号--序号   共64位
        long longStr = ((timestamp - twEpoch) << timestampLeftShift) | (dataCenterId << dataCenterIdShift) | (workerId << workerIdShift) | sequence;
        return longStr;
    }

    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    protected long timeGen() {
        return System.currentTimeMillis();

    }
}
